Kiev , Ukraine -LRB- CNN -RRB- -- A perilous face-off intensified Saturday when Russia state news complained that Ukraine had mobilized 15,000 troops in the suburbs of Slavyansk in eastern Ukraine `` in order to wipe out the city and its residents . ''

Quoting a Russian Defense Ministry source , RIA Novosti said satellite photos showed the force forming around the city that has become a friction point between the Ukraine military and pro-Russian militants .

The Defense Ministry source said the number of Ukraine troops put the pro-Russian militants at a disadvantage because the latter are `` armed only with small amount of pistols and shotguns . '' Many eastern Ukraine residents have Russian roots and sympathize with Moscow .

The source said the photos showed about 160 tanks , 230 infantry combat vehicles and armored personnel carriers , mine throwers and multiple-launch rocket systems .

Russian President Vladimir Putin has repeatedly criticized Kiev 's use of force against Ukrainian civilians .

Developments in Ukraine have come at a rapid pace in recent days :

-- Russia , which already had 40,000 troops on its side of the border , started new military drills a few days ago after Ukrainian forces said they killed five pro-Russian militants . Ukraine launched the second stage of an `` anti-terrorist operation '' against militants in Slavyansk .

-- On Friday , a team of European and Ukrainian military observers were seized Friday by pro-Russian separatists in Slavyansk .

-- Russian military aircraft `` crossed and violated '' Ukrainian airspace seven times overnight , Ukrainian Prime Minister Arseniy Yatsenyuk told reporters in Rome on Saturday . The Russian Defense Ministry denied the accusation , according to the state news agency Itar-Tass .

-- Yatsenyuk met with Pope Francis while in Rome on Saturday . The meeting has been seen as a sign of support from the Vatican for his government .

-- G7 leaders said they would impose new sanctions on Russia over its role in the crisis .

The Ukrainian Prime Minister urged Russia to pull back its security forces and not to support pro-Russian militants in eastern and southern Ukraine . `` We urge Russia to leave us alone , '' he said in televised remarks .

Ukraine 's government has promised constitutional reforms and protections for Russian speakers in a bid to ease the tensions in its eastern regions .

Inspectors seized in Slavyansk

On Saturday , the fate of the military inspectors preoccupied world leaders .

The inspectors from the Organization for Security and Co-operation in Europe were detained Friday as they entered Slavyansk , along with five Ukrainian military representatives and the driver of their bus , Ukraine 's Interior Ministry said .

Ukraine 's Security Service , the SBU , said the group is being kept under `` inhumane conditions '' in the basement of a building held by the militants .

The self-declared mayor of Slavyansk , Vyacheslav Ponomarev , told reporters that one of the `` prisoners '' has diabetes , but he has the medicine he needs and will be given his own quarters overnight .

Separatist leader Denis Pushilin , self-declared chairman of the so-called `` Donetsk People 's Republic , '' told CNN he does n't believe they are from the OSCE , but that some are NATO spies .

The German Foreign Office said it had set up an emergency task force to find out what has happened to the team members , four of whom are German . The others are from Denmark , Poland , Bulgaria and the Czech Republic , Russian state media said .

The OSCE mission in Ukraine is tasked with helping to implement an international agreement signed nine days ago in Switzerland , which called for illegal militia groups to disarm and leave occupied buildings , among other provisions .

In a phone call with U.S. Secretary of State John Kerry , Russian Foreign Minister Sergey Lavrov asked the United States to use its influence to secure the release of pro-Russian leaders being held in Ukraine .

Kerry urged Russia to support efforts of the OSCE and the government of Ukraine to liberate the inspectors and their Ukrainian guides , according to a senior State Department official .

Targeted sanctions

Against the backdrop of increasing volatility in Ukraine , leaders of the G7 industrialized nations on Friday announced they would `` move swiftly to impose additional sanctions on Russia '' over its actions in Ukraine .

The statement from the group -- which includes Canada , France , Germany , Italy , Japan , the United Kingdom and the United States -- came hours after U.S. President Barack Obama threatened Russia with new sanctions .

CNN 's Gul Tuysuz reported from Kiev and Laura Smith-Spark wrote and reported from London . CNN 's Andrew Carey and Nick Paton Walsh in Slavyansk and journalist Victoria Butenko in Kiev contributed to this report . CNN 's Alex Felton , Bharati Naik , Ben Brumfield and Boriana Milanova also contributed .

@highlight

Russian state news says 15,000 Ukrainian troops in eastern Ukraine

@highlight

Self-declared mayor of Slavyansk says diabetic hostage has the medicine he needs

@highlight

Ukraine 's Security Service says OSCE team is being held in `` inhumane conditions ''

@highlight

U.S. official says planned U.S. sanctions `` can have a significant impact '' on Russia